Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image or map of functional processes in a living organism. A typical PET imaging system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule such as, for example, fluorodeoxyglucose. After enough time has elapsed for the tracer to distribute and concentrate within certain tissues, the patient is scanned to reveal the tracer's whereabouts. When a positron is emitted by the tracer it travels only a few millimetres before it annihilates with an electron. The annihilation event produces a pair of 511 KeV gamma photons that travel in nearly opposite directions (180°±0.23°) from each other. These photons are absorbed by a circumferential ring of scintillators that create bursts of visible light photons that, in turn, are detected by photodetectors. PET imaging systems are thus based on coincident gamma photon detection; photons not arriving in pairs being ignored. Because each pair of detected gamma photons travels in nearly a straight line (called the line of response (LOR)), the tracer's location may be determined by identifying LOR intersections (“sweet spots”).
The spatial resolution of these sweet spots degrades significantly toward the periphery of the field of view (FOV), and is characteristic of all conventional ring based PET imaging systems. This non-symmetric broadening of the tracer point source is a result of assigning detected interaction events to the wrong LOR (see FIG. 1). Thus, and in order to improve peripheral image resolution, the position or depth of gamma photon interactions occurring within the scintillators (of the ring of scintillators) needs to be accurately determined. PET imaging systems that provide depth of interaction (DOI) information can correctly position interaction events thereby resulting in more uniform resolution throughout the FOV.
A number of methods for extracting DOI information from a PET detector module have been proposed; however, conventional PET imaging systems do not provide adequate DOI information. A drawback to a number of the proposed systems is the requirement for additional detector electronics. In one very promising approach, however, light sharing along the long length of a pair of optically coupled scintillation crystals is used to extract DOI information based on the ratio of light outputs from each of the paired crystals. More specifically, DOI information may be extracted from the ratio of light collected between neighbouring crystals (A and B) of a crystal pair using simple Anger type logic [A/(A+B)]. Although initial work in this area has been promising, practical implementation for small cross section crystals and optimization of the light sharing between crystals has been limited due, in large part, to significant light sharing in the glass envelope of conventional position sensitive photomultiplier tubes (PMTs). (R. S. Miyaoka, T. K. Lewellen, H. Yu, D. L. McDaniel, Design of a Depth of Interaction (DOI) PET Detector Module, IEEE Trans. Nucl. Sci., Vol. 45, No. 3, June 1998 pp: 1069-1073; T. K. Lewellen, R. S. Miyaoka, DMice—a depth-of-interaction detector design for PET scanners, Proceedings of the IEEE Nuclear Science Symposium and Medical Imaging Conference, Rome (2004): 2388-92).
Accordingly, there is a need in the art for new and improved scintillation detectors capable of detecting the position or depth of gamma photon interactions occurring within a scintillator. The present invention fulfils these needs and provides for further related advantages.